Ship exhaust gas greatly threatens human health and the environment. The ozone/urea system has the advantages of strong oxidability, excellent reducibility, and low reductant consumption. This paper thoroughly studied the effects of urea concentration, O3 concentration, initial pH, temperature, O2 concentration, SO2 concentration, and initial NO concentration on NOx removal efficiency using the ozone/urea system. Moreover, this paper conducted a systematic study on the conversion proportion of NOx into nitrate and nitrite, which is very important for the ship NOx wet method. Urea concentration and O3 concentration played important roles in NOx removal efficiency. Weak acid and weak alkali media could effectively improve NOx removal efficiency, but the reasons were completely different. As the temperature increased, the NOx removal efficiency initially decreased and subsequently increased. Besides, the increase of temperature and initial pH could decrease the conversion proportions of NOx into nitrate and nitrite. The increase of SO2 concentration could not only reduce the NOx removal efficiency but also increase the conversion proportion of NOx into nitrate. However, increasing SO2 could significantly decrease the conversion proportion of NOx into nitrite. Under the optimal conditions of this paper, the NOx removal efficiency reached 97.8% and the conversion proportion of NOx into nitrate was only 20.39%. In addition, the urea content was just 0.3 mol/L, which was far lower than that in the prior study. This illustrated that the ozone/urea system had excellent application potential in marine exhaust gas purification.